You mentioned no mechanical ability, yet you want to get a kit, but want to buy a complete item and hand assemble it yourself. You really ought to just buy a Crystalyte kit from electricrider.com in Kansas, and put the thing together yourself. It is very reliable, and depending on your configuration, you can get it as fast as 50mph, unless that is too fast for you. The lightness will be determined by the type of batteries you choose to go for. NiMH is becoming a very good option for price and weight. Lithium is the lightest, but pricey.
Crystalyte has a throttle, and you can just let the motor and battery and controller do all the work if you want. That is fine. Keep in mind if you go this route, lightness will suffer, as you will need a bigger battery, to run the full length of your commute. Otherwise, pedal some way, take a break. Bike some more. The best value is to get a kit and install it on your already high quality bike. Otherwise you will be getting a cheap grade bike. You want reliable right?
Stick with a hub motor if you want stealthy.